Fluorescent labeling can improve the detection sensitivity in capillary electrophoretic (CE) separations down to attomolar concentrations. However, most fluorescent labels are not compatible with CE because their fluorescence properties and charge states are pH-dependent, they are often hydrophobic, and they have a tendency to significantly change the properties of the analytes after labeling. Further, to achieve labeling of analytes in a timely manner, it is necessary to use high concentrations of analyte (e.g., preconcentrated analyte). To address this latter problem, the immobilization of enzymes and derivatizing reagents on solid phases has been studied by several groups. The main motivation for doing these reactions on a solid phase was the observation that reactivity could be increased when the reagents were immobilized compared to when the same reactions were carried out in solution. Unintended multiple labeling of an analyte can still be a problem with this approach, however.